Last week, our design team saw a prime example of why you should always get multiple quotes. A customer questioned why our bid’s estimated solar production wasn’t significantly higher than that of another company’s bid, despite their design being over a kilowatt smaller.
Initially, we were stumped. We use the same modeling software, so we have access to the same shade data, but we couldn’t replicate their numbers. Finally, we disabled the LIDAR shading in our model, and bingo, everything lined up.
This is a problem. Unless your house is in the middle of a field, you’re going to have some shading. It’s a fact of life, especially in western Washington. And that needs to be factored into your system design, not only for system efficiency and functionality, but in order to meet your goals. If your system was designed to offset 100% of your usage, you’d be pretty disappointed to discover that, in real world conditions, it only covers 85%.
What can this look like?
Take this house, for example. It faces southeast and is partially shaded by the neighbor’s massive magnolia tree. Taken at face value, the first model looks like the best bet: it’s the smaller of the two system sizes, but produces almost as much as the larger system. From a numbers standpoint, it seems like the best bang for your buck. But once you factor in shading on that design, that number looks very different.
Modeled without LIDAR shading
System size: 12.88 kW
Annual production: 14,669 kWh
Modeled with LIDAR shading
System size: 12.88 kW
Annual production: 13,290 kWh
Modeled with LIDAR shading
System size: 14.26 kW
Annual production: 14,959 kWh
Are there times when it makes sense to disable LIDAR shading? Absolutely! If nearby trees have recently been topped or removed, or if the home is new construction on a wide-open lot, we adjust our model to suit. The difference is, we have that conversation with you, so you’ll always know how we arrived at our numbers.
When should you dig deeper?
Typically, you can expect your quotes to be similar in size and production, even accounting for different equipment. If you see an outlier, ask how they’re factoring in shading. They should be able to provide you with a report detailing the impact of shading on your roof.
You can also do your own quick calculation based on average production for our region. The National Renewable Energy Laboratory, NREL, calculates that systems in western Washington produce around 1100 kWh per kW installed. Multiply your system size (in kW) by 1100 and that should get you in the ballpark. This doesn’t account for on-site shading, roof pitch, or azimuth, so for most systems, you can expect your production estimate to fall below this number.
We hope this information helps you on your solar journey! As always, do your homework, get multiple bids, and never be afraid to ask too many questions.